Consolidate StockFit portfolio evidence

[FLABDUL]

Summary

• combine the point-in-time backtest and twelve-company data-quality audit into one recruiter-facing case study
• update regression evidence to 424 passed, 3 skipped and 12 deselected
• embed the tracked NAV comparison as the primary visual and the audit chart as supporting evidence
• retain explicit limitations and the unpublished status

Verification

• factual claims checked against committed live summary JSON
• Ruff 0.16.8 check and format check pass
• Pyrefly: 0 errors
• pytest: 424 passed, 3 skipped, 12 deselected
• doctest: 30 passed
• credential scan found no credential; documentation-only pattern examples remain

Known local limitation

The strict HTML documentation build hits the repository's pre-existing Windows case-insensitive autosummary collision between Portfolio.Cash and Portfolio.cash. The edited Markdown files are outside the Sphinx source tree; Linux CI is the authoritative docs build.